The two attended a pre-Oscars party at the Chateau Marmont, and then left TOGETHER. According to witnesses at the party, the two actors were all loved up, and “even kissed” at the party.

Looks like there may be trouble in paradise for Miley Cyrus and Liam Hemsworth!

The Australian actor, who got engaged to the “Can’t Be Tamed” singer in June 2012, cheated on his famous fiancee by getting hot and heavy with Mad Men star January Jones at a pre-Oscar party over the weekend, the new issue of Star is exclusively reporting.

Liam, who showed up at the swanky soiree at the famed Chateau Marmont hotel without his bride-to-be, had guests aghast as he cozied up to the scandalous single mother, according to Star.

“It looked like they had some hot chemistry,” an eyewitness at the event tells Star. “They were all over each other – and they even kissed!”

Meanwhile, Miley, who has been gushing about her relationship with the Hunger Games star in several recent interviews, even referring to him as her “hubby,” hit up the Elton John AIDS Foundation bash on Sunday night solo, where she was “all over the place,” a source tells Star.

One thing that strikes me as very odd in all of this, is how silent Miley has been this week. She hasn’t Tweeted a word since Monday which is incredibly unlike her (especially when it involves her man). I’m thinking SOMETHING has happened, otherwise she’d be talking about it. I dunno, methinks there’s trouble in paradise!